The present invention relates to an improvement on prior U.S. application, Ser. No. 340,486, filed Mar. 12, 1973, now abandoned, which is similar to corresponding British patent specification No. 1,435,444.
Prior to the present invention it was generally known to provide a pneumatic conveying device consisting of a screw impeller rotatably mounted within the bore of a casing for transporting material from a source such as a hopper flow connected to the inlet of the casing through the bore to a casing outlet. Material is discharged from the casing outlet into a discharge chamber or wind box. Gaseous fluid under pressure is supplied to the discharge chamber for conveying material out of the discharge chamber and through a pneumatic conveying line.
Early designs of pneumatic conveying apparatus of the type to which the present invention relates were generally as shown in U.S. Pat. No. 1,677,119, issued July 10, 1928. This design employed a front and rear bearing to rotatably support the screw impeller at both ends. A compressing screw formed by either a reducing pitch screw or a reducing diameter casing bore or an increasing diameter screw impeller shaft or a combination thereof served to compact material within the casing bore to thereby form a material seal in the casing bore between the material outlet of the casing and the material inlet of the casing. This material seal served to prevent "blow back" from the discharge chamber into the feed material hopper, i.e. prevent air under pressure from short circuiting into the material feed hopper through the bore in the casing rather than conveying material out of the discharge through the pneumatic conveying line.
A later development in pneumatic conveying apparatus to which the present invention relates is generally shown in U.S. Pat. No. 2,299,470, issued Oct. 20, 1942. In that apparatus, the front bearing supporting the screw impeller of the earlier design was eliminated. An overhung screw impeller supported by a rear bearing is employed. The front end of the screw impeller is kept centered within the bore or pump barrel by employing a heavy impeller shaft and by keeping the pump barrel full of material.
In both the design of U.S. Pat. No. 1,677,119 and U.S. Pat. No. 2,299,470, the material outlet was coextensive with the end of the pump barrel or bore through the casing. In the design of U.S. Pat. No. 1,677,119, the outlet of the casing is immediately upstream of the source of conveying gas and the beginning of the conveying line. In U.S. Pat. No. 2,299,470, the outlet of the casing discharges material into a wind box or discharge chamber. This discharge chamber has an outlet connected to a pneumatic conveying line. Air under pressure is supplied through jets into the discharge chamber for entraining material and conveying it through the conveying line.
A further difference between the design of U.S. Pat. No. 1,677,119 and the design of U.S. Pat. No. 2,299,470 is the use of a normally closed flap valve in the later patent for closing the outlet of the casing. During initial start-up of the pump, this valve serves to assist the compressing screw in forming a material seal at the outlet to prevent "blow back" of air through the pump casing. Once the material seal is formed, the flap valve will swing into the discharge chamber away from the casing outlet and remain open so long as material is being conveyed through the casing.
Although the device of U.S. Pat. No. 2,299,470 resulted in improved operation due to the addition of the discharge chamber and flap valve, the lack of a front bearing for supporting the screw resulted in maintenance problems with the screw due to the overhung design. This is particularly true with large capacity apparatus where the screw is heavy and when the apparatus is operated at less than capacity. As long as the apparatus is operated at capacity, the material in the casing bore will assist in supporting the front of the screw. When the apparatus is operated at less than capacity, the support provided by the material is no longer available. The lack of front end support of a heavy screw permits the screw to droop. As the screw rotates, it will whip causing the screw flights to make excessive contact with the casing bore thereby reducing the life of the screw and/or the pump barrel lining. Each time the wear on the flights or the barrel lining exceeds a given amount, the screw and/or barrel lining must be changed. This causes a shut-down of the equipment.
Therefore, each of the prior apparatus had its advantages and its disadvantages. The front and rear bearings provided the advantage of two supports for the conveying device screw, but the disadvantage of difficulty of establishing a material seal during start-up. The second generation had the advantage of ease of establishing a material seal during start-up, but the disadvantage of reduced wear life of the screw due to the single support for the screw.
Various attempts have been made to combine the advantages of using both front and rear bearings for supporting the screw impeller and a valve at the discharge of the pump casing. Such attempts are shown, for example, by U.S. Pat. Nos. 1,545,230; 2,489,980; 3,314,733; and 3,704,917. None of these apparatus have become commercially successful. Each presents problems such as the inability of the valve at the outlet of the casing to operate over long periods of time without interfering with the operation of the apparatus or becoming fouled due to its direct placement within the flow of material. An attempt to provide a double support screw impeller while combining a flap valve to establish a material seal during start-up was set forth in U.S. patent application Ser. No. 340,486, filed Mar. 12, 1973. This apparatus employs a first section of a screw impeller having a standard compressing screw, i.e. screw flights which reduce in pitch toward the casing outlet, and a second screw section with reverse flights for preventing material from entering the forward bearing. The material outlet from the casing is at right angles to the bore through the casing and a plate interconnects the two screw flights at the casing outlet. It was found during testing of this apparatus that the flap valve would constantly open and close with great rapidity producing a constant banging of the flap valve with the result that the flap valve tended to shake itself apart and conveying capacity was affected. A further result was that the expected increased wear life of the screw impeller and device as a whole did not materialize.
Other prior attempts at providing a pneumatic conveying apparatus employing front and rear bearings for supporting a screw impeller with an outlet intermediate the bearings and at an angle to the bore in the casing are shown in U.S. Pat. Nos. 2,132,980 and 2,393,412. The apparatus illustrated in these patents are designed for low capacity, short distance conveying and are not adapted to provide an adequate seal for conveying high capacities over long distances.
With prior apparatus of the type to which the present invention relates, the amount of horsepower required to turn the screw impeller was often greater than desirable. It is believed that this is at least partially due to the requirement that with prior apparatus, the impeller was rotated within compacted material and a material seal of excessive length is required.